What concept explains why a farmer exposed to cowpox hasn’t contracted smallpox?

The concept that best explains why a farmer exposed to cowpox hasn't contracted smallpox is cross-immunity. Cowpox and smallpox are both caused by viruses that are similar in structure and function, specifically belonging to the same family of viruses known as the Poxviridae. When a person is infected with cowpox, their immune system produces antibodies and activates immune cells that can recognize and respond not only to cowpox but also to smallpox due to the similarities between the two viruses. This immune response provides a protective effect against smallpox, hence the farmer remains immune after exposure to cowpox.

While immunization is related to the protective effect achieved through exposure to weakened or similar pathogens, the farmer’s immunity in this case operates through natural infection with cowpox leading to cross-immunity against smallpox. Therefore, cross-immunity is the most accurate description of what is occurring in this scenario.